Scanners represent a low cost and convenient way to capture images and documents. Devices that give 300 dots-per-inch (dpi) or higher optical resolution are readily available and are more than adequate to acquire documents containing text in the size of 8 points or even 7 points. By contrast, some digital camera imaging devices have a VGA resolution of 640×480 pixels. If applied to imaging text on a U.S. letter-sized page (i.e., 8.5 by 11 inches or approx 21.5 by 28 cm), such a VGA camera is capable of achieving only about 60 dpi. Such resolution is inadequate to distinguish text characters used in most text documents.
A drawback of scanners is that they are often slow; scanning a page can take a significant fraction of a minute because of the linear motion between the page and the scan bar. This makes acquisition of multi-page documents particularly inconvenient and time-consuming. By contrast, a VGA video camera is generally capable of acquisition at 60 frames/second so that acquisition of multiple image frames is comfortably achievable in less than a second.
The present invention uses a digital imaging device such as a digital camera to obtain multiple digital images of all of a text document segment. In one implementation, the text document segment could be the entire text document so that each digital image would be of the entire text document. The multiple images are obtained while lateral jittering is imparted between the digital imaging device and the text document. The lateral jittering may be in a pair of transverse (e.g., perpendicular) directions. An enhanced resolution representation of the text document is formed from the multiple laterally displaced images and de-blurred as a multi-frame reconstruction.
The multi-frame reconstruction can be significantly simplified because text documents primarily include regions that are spatially piecewise constant (e.g., monochrome text on a monochrome background). In contrast, conventional multi-frame image reconstruction addresses the issue of reconstruction of arbitrary image objects, which requires solution of ill-constrained Fourier reconstruction problems.
Directing consideration to images that are spatially piecewise constant means that only transitions between a limited palette of discrete values need be ascertained. This simplification holds well for plain text on plain backgrounds and greatly eases the task of reconstruction from multiple frames. In one exemplary implementation employing a digital imaging device (camera) with VGA resolution, this multi-frame reconstruction algorithm can achieve a 4× improvement in resolution in each dimension to provide an equivalent of a 240 dpi resolution scan in a second or so.
Additional objects and advantages of the present invention will be apparent from the detailed description of the preferred embodiment thereof, which proceeds with reference to the accompanying drawings.